1. Field of the Invention
The present invention relates in general to the field of information processing, and more specifically to a system and method for interleaving and inverting multi-channel pulse width modulated signals.
2. Description of the Related Art
Signal processing systems utilize pulse width modulators (PWMs) in many signal processing applications. Pulse-width modulators generally modulate a duty cycle of a nearly pure square waveform to vary an average value of the waveform. FIG. 1 depicts a multi-channel audio system 100 that utilizes multiple PWMs 102.0 through 102.M (collectively referred to as PWMs 102) to drive associated half-bridge amplifiers 104.0 . . . 104.M. “M” is an integer greater than one (1), and M+1 represents the number of channels in the multi-channel audio system 100. Channels 0 . . . M represent respective signal paths in multi-channel audio system 100. For example, a 5.1 multi-channel stereo system has 6 channels for six respective speakers.
A multi-channel digital signal source 101, such as a multi-channel audio signal processing system, provides M digital input signals x0(n) . . . xM(n) to the PWMs 102. Input signals x0(n) . . . xM(n) represent data for the respective 0 . . . M channels of multi-channel audio system 100. The PWMs 102 respectively convert the digital input signals x0(n) . . . xM(n) into pulse width modulated output signals u0, . . . , uM.
Half-bridge amplifiers are ubiquitous and, in general, switch an output signal between two voltage levels such as +V and VREF. “+V” generally represents a voltage source that can be any value and, in power applications, can range from, for example, +15V to +100V. VREF is less than +V and is often ground or −V, e.g. −15V to −100 V. Half-bridge amplifiers 104 conceptually illustrate the switching nature of half-bridge amplifiers in general. Each of the half-bridge amplifiers 104 includes two conceptual groups of switches 106 and 108. Switches 106 and 108 are complementary; thus, switches 106 conduct when switches 108 are in an essentially open, high impedance state and visa versa. When the pulse width modulated output signals u0, . . . , uM are logical ones, switches 106 conduct and drive half-bridge output signals p0 . . . pM to approximately +V. When the pulse width modulated output signals u0, . . . , uM are logical zeros, switches 108 conduct and drive half-bridge output signals p0 . . . pM to approximately VREF and cause switches 106 open. Half-bridge amplifiers 104 can be implemented in many ways using, for example, power transistors and diode bridge networks.
The low pass filters 110.0 through 110.M respectively average the half-bridge output signals p0 . . . pM to respectively generate respective continuous time audio output signals y0(t) . . . yM(t) of respective audio channels 0 . . . M. Audio output signals y0(t) . . . yM(t) drive speakers 112.0 . . . 112.M to produce audio frequency sound waves.
FIG. 2 depicts multi-channel pulse width modulated output signals u0, . . . , uM 200. Pulse width modulated output signals u0, . . . , uM have respective rising edges at time t0, t2, t4, and t6, . . . , such as rising edges 202.0 . . . 202.M, and respective falling edges at times t1, t3, t5, and t7, such as falling edges 204.0 . . . 204.M. Each of the output signals u0, . . . , uM is a series of frames with each frame having a period T equal to ti−ti+1. Each frame of output signals u0, . . . , uM has a respective PWM pattern. The duty cycle of each frame of output signals u0, . . . , uM equals the pulse width duration divided by the period T times 100%. The duty cycles of output signals u0, . . . , uM depicted in FIG. 2 are each 50%, which is representative of a very low-level signal.
When PWMs are operated, there is a significant issue with electromagnetic interference (EMI). The sum of the pulse voltage amplitudes of the output signals u0, . . . , uM at any given time is directly proportional to the total EMI power of multi-channel audio system 100. These radio frequency (RF) signals are created by the large voltages and currents that are switched at moderate frequencies. These voltages may be 30 volts, 5 amps switched at 384 kHz, as an example. It is difficult to shield this RF energy from leaking into the rest of the multi-channel audio system 100, other adjacent systems, or violating governmental regulatory EMI standards. At high signal levels, the amount variation of the pulse edges over time reduces the peak EMI levels at any given time, tempering the situation to some extent. At low level audio signals, the PWM signals are nearly pure square waves. As these square waves are at 250-500 kHz in most applications, harmonics of the signals can be very strong in the MHz region. AM radio, and even FM radio, reception can be compromised.